1. Field of the Invention
This invention relates to an electrostatic powder coating method, and more particularly to a method of selectively coating a plate with electrostatic powder so that the parts thereof on which the coating is not necessary or the coating is undesirable are not applied with coating.
2. Description of the Prior Art
The electrostatic powder coating method is well known in the art and has various advantages over the conventional coating method by use of solvent type coating material. In the electrostatic powder coating method, the charged powders employed are electrostatically attracted by the charged plate and fixed thereto by fusing. Since no solvent is used, there is no problem of pollution and it is possible and easy to apply a thick coating on the plate. Further, the characteristics of the coating are superior to those of the solvent coating process.
On the other hand, it is well known to provide marking lines on a plate by an electrophotographic method in the field of shipbuilding, building construction, civil engineering etc. As is well disclosed in Japanese patent publication No. 12385/1969, the electrophotographic marking method is a process to record marking lines on a plate by modulating the electrostatic attractive force between the plate and the photoconductive powder applied thereon in accordance with pattern information based on an optical original.
In general, the plate or member on which the marking lines are provided is cut or welded along the marking lines. The marked area of the plate should be free from a coating material since the coating material hinders cutting and welding of the plate. In welding, since the plate to be processed is heated up to a high temperature, the coating material is burnt or vaporized and blowholes or pits are made in the bead weld and the welding strength is lowered thereby. Therefore, the part of the plate to be welded is best be free of coating material. After uniform application of the coating material, the coating material is desirably removed from the surface of the plate. Also in cutting, the coating material existing on the plate along the cutting line hinders smooth cutting of the plate at high speed. Further, the material coating the plate is damaged by heat when the plate is cut, and accordingly, the plate must be coated with the coating material again after the cutting.
The area in which the coating is not necessary or undesirable is often quite large. Therefore, if it should be made possible to apply the coating material only in the range where the coating is necessary, there would be a considerable saving of labor and lowering of the cost of the coating process.
In the conventional coating process using a solvent type coating agent, it has been known to apply the coating material in a pattern-like area on a plate by spraying the coating material on the plate with a masking member placed thereon to selectively cover the plate in a pattern-like form. However, this type of selective coating is very difficult to accomplish in the electrostatic powder coating process. This is because the powder which is electrostatically applied on the plate to be coated is not fixed to the plate until it is fused by heat, and accordingly, the powder is likely to fall on the plate in the region where no coating is desired when the masking member is removed from the plate with the powder applied thereon. The powder which falls on the plate in the area where no coating is desired is also fixed to the plate when the plate is heated to fuse and fix the powder thereon. If the powder is heated for fixing without removing the masking member, the powder on the masking member is also fused and fixed to the member and there results a great loss of costly coating powder. Further, in case that the masking member is of large size, it is difficult to remove the masking member from the plate without scratching or damaging the surface of the plate.
Further, it has also been known in the art to apply the coating material in a pattern-like area on a plate by controlling the operation of the spray gun for spraying the electrostatic powder on a plate. This is, however, very impractical and difficult, since the charged coating powder moves along electric lines of force between the spray gun and the surface of the plate which radially extend therebetween. The charged powder, therefore, is distributed widely over the plate and the thickness of the coating material on the plate is not uniform.
Another example of a method of selectively applying a coating material in a pattern-like area on a plate is to utilize an electrophotographic method. As shown in Japanese patent publication No. 22645/1963 and British Pat. No. 990538, photoconductive powder can be selectively removed from the surface of a plate after it is uniformly applied thereon by exposing the plate with the photoconductive powder to imagewise light. By the imagewise exposure, the photoconductive powder which has been charged and fixed to the surface of the plate by electrostatic force is released therefrom. After the photoconductive powder is selectively removed, electrostatic coating powder which is charged in the same polarity as that of the photoconductive powder is scattered on the plate so that the latter may fall on the plate only in the area where the photoconductive powder does not exist. Then the coating powder is fused and fixed to the plate and the photoconductive powder is removed from the plate after uniform exposure of the plate to light. Thus, a pattern-like coating can be obtained on a plate.
The above described method of forming a pattern-like coating on a plate by use of photoconductive powder suffers from a defect that the coating powder mingles into the photoconductive powder which is recovered. The properties of the photoconductive powder are degraded by the coating powder mingled therewith. Therefore, in the method of using the photoconductive powder, the coating powder must be separated from the recovered photoconductive powder.
It is difficult to perform the separation with high efficiency and obtain photoconductive powder of high purity. The whole process becomes complicated owing to this separation step and the cost of the coating process is consequently not lowered. The main reason for the difficulty in separation is that the photoconductive powder and the coating powder aggregate together by friction charging. Since the coating powder is charged in advance, the static aggregation force is quite large. Besides, since the particles of the photoconductive powder and the coating powder are very small in size, i.e. 20 to 150 microns in diameter, and the two kinds of powder are very similar in appearance, the separation of the two is very difficult in mechanical sense.
The foregoing method of forming a pattern-like coating on a plate by use of photoconductive powder is further disadvantageous in that the properties of the coating powder are degraded by the mingling of the photoconductive powder into the coating powder which occurs when the photoconductive powder is removed from the surface of the plate. Further, the properties of the coating powder are degraded by the fog formed in the background of the pattern when the photoconductive powder image is formed on the plate. This method of forming a pattern-like coating on a plate is further disadvantageous in that the thickness of the coating obtained by one coating process is limited by the surface potential of the photoconductive powder layer formed on the plate prior to the coating step. This is because the coating powder will stick on the photoconductive powder layer also if the surface potential of the coating powder becomes higher than that of the photoconductive powder.